1. Field of the Invention
The present invention relates to a waveguide-type optical circuit provided with an optical coupler such as a directional coupler.
2. Description of the Related Art
In a conventional Directional Coupler (DC), as described in Paragraph 0024 in International Publication Pamphlet No. WO 2006/075702, a region (gap portion) sandwiched between two waveguide cores at a close portion of waveguides is supplied with a less amount of glass particles during formation of upper cladding because waveguide patterns are close to each other, and the density of the glass particles therefore becomes sparse. Meanwhile, a region (non-gap portion) that is not sandwiched between the two waveguide cores is sufficiently supplied with the glass particles. Therefore, the densities of the glass particles in the gap portion and the non-gap portion become different from each other. After the upper cladding is formed, stresses to incline the two waveguide cores toward inside the gap portion occur in the two waveguide cores. Consequently, optical major axes of the two waveguide cores become inclined while polarization mode coupling occurs.
Therefore, a Polarization Dependent Loss (PDL) caused by the polarization mode coupling in DC occurs in the DC or in a Mach-Zehnder Interferometer (MZI) circuit that uses the DC. An optical branch coupler such as the DC includes a Planar Lightwave Circuit (PLC)-type optical coupler such as a PLC-type 2×2 coupler. The MZI circuit using the DC includes a PLC-type variable optical attenuator. However, there is a problem in that large PDL prevents the PLC-type optical coupler and the PLC-type variable optical attenuator or the like from becoming widespread. In addition, in optical branch couplers having a structure with closely positioned waveguide cores such as a Multi-Mode Interference (MMI) coupler and a Y branch, a phenomenon similar to the DC occurs, and thus, PDL caused by the polarization mode coupling occurs also in the MZI circuit that uses the optical branch coupler such as the MMI coupler and the Y branch.